An electric motor is used as a power source for a hybrid car in which the electric motor is combined with an engine, an electric car, or the like. To drive the electric motor, an inverter is used to obtain a predetermined torque and a predetermined frequency. The inverter is incorporated in the car, and there has been a demand for a reduction in the size of the inverter and an increase in the power of the inverter in order to provide an on-board space.
In a hybrid car in which the operating temperature of the inverter varies significantly depending on the driving environment of the car, particularly in which the inverter is mounted in an engine compartment, the inverter becomes very hot as a result of heat generated by the engine. The temperature of a switching element in the inverter is raised not only by the above-described ambient temperature but also by a steady-state loss resulting from the flow of a current through the switching element itself and a switching loss resulting from switch-on and switch-off operations. The switching element may be destroyed if the temperature exceeds a certain value.
In order to avoid destroying the switching element, a method has been proposed in which the temperature of the switching element is detected so as to allow the inverter to be cooled based on the information obtained or in which the temperature of the switching element or inverter is measured to allow a torque or a switching frequency to be limited. A voltage of a PN junction semiconductor element such as a diode varies linearly depending on temperature. Accurate, highly responsive temperature information is obtained by observing the voltage of a diode installed near the switching element as a temperature detection sensor. The accurate temperature information obtained allows the torque to be output until the temperature of the switching element becomes close to its breakdown temperature. This is expected to enable an increase in the density of inverters.
In Patent Document 1, the torque limitation is started when the temperature of the switching element exceeds a certain value and is varied in proportion to the raised temperature and a temperature change rate.
This technique allows the switching element to be prevented from generating heat, thus avoiding destruction of the switching element.    Prior Technical Document    Patent Document    Patent Document 1: Jpn Pat. Appln. KOKAI Publication No. 10-210790